The fan casing of a gas turbine aircraft engine can fulfil several functions. It defines the air inlet flow path in the engine, supports abradable material relative to the tops of fan blades, supports a structure generally in the shape of acoustic panels for attenuation of sound waves at entry of the engine and incorporates or supports a retention shield for debris projected onto the inner face of the casing, such as ingested objects or debris from damaged blades.
Currently, a fan casing comprises a relatively thin wall defining the air inlet flow path. The casing can especially be made of metal or composite material. For example, it has been proposed in document FR 2 913 053 to make the fan casing of composite material of variable thickness by forming a fiber reinforcement and densification of the reinforcement by a matrix. The fiber reinforcement is formed by winding a fiber texture obtained by three dimensional weaving with varying thickness on a mandrel in superposed layers so as to integrate the retention shield by simple localized thickness increase.
The fiber reinforcement comprises fibers, especially carbon, glass, aramid or ceramic. The matrix as such is typically a polymer matrix, for example epoxide, bismaleimide or polyimide.
The casing can be made in a single piece and comprises flanges at its axial ends. A first flange, called upstream flange, enables fastening of an air inlet sleeve on the casing, while the second flange, called downstream flange, enables connection of the fan casing with an intermediate casing by means of linking members of screw-nut type, with interposition of an annular ferrule applied against the downstream face of the downstream flange. Here, the upstream and the downstream are defined by the direction of flow of gases in the turbine engine. The intermediate casing is made of metal, titanium, in a metal alloy based on titanium or even made of aluminium, along with the annular ferrule and the air inlet sleeve.
In usage, it eventuates that some cases of breakdowns can cause a rise in the temperature surrounding the fan casing, which can harm its proper operation, especially in the event where the fiber reinforcement is formed from a three-dimensional fiber texture. In fact, material currently used for casings made of composite material is not intrinsically self-extinguishable or flame-retardant. Also, fibers oriented in the third direction of the reinforcement maintain the structure and limit its delamination. However, these fibers oriented in the third direction transmit heat of the flame within the material, which benefits maintaining of the flame.
It is therefore necessary to protect the fan casing against these extreme temperatures and more generally against fire by limiting heating of the structure to allow its extinguishing so that the casing is capable of conserving its mechanical properties during and after the rise in temperature and if needed slow down the spread of the fire following withdrawal of the flame.
It has therefore been proposed to connect counter plates made of titanium, a material less sensitive to fire, between the fan casing and the adjacent upstream and downstream pieces. But, this solution increases the overall mass of the engine. Also, production tolerances are considerable due to the production method (hot-forming of the counter plate), which is also difficult to carry out and therefore very costly.
It has also been proposed to connect, by weaving on the preform intended to form the fiber reinforcement, a rigid or semi-rigid braid made of glass fibers, which is then co-injected with the preform of the fan casing. However, this solution proves delicate in terms of maintaining protection during its introduction into the mold. Also, the weaving step proves difficult to perform.
It has also been proposed in document GB 2 486 404 to hot-project copper or a copper alloy onto the outer surface of the barrel of the casing made of composite material so as to diffuse the temperature of the flame and protect the casing against fire.
Finally, document EP 2 447 508 proposes connecting panels made of flame-retardant or insulating material so as to protect a part of an engine. For this purpose, the panels are attached and fixed by way of fastening means to the surface of the part to be protected. But this document fails to specifically address the problem of casings made of composite material.